This invention relates generally to buckling beam probes for testing integrated circuits. More particularly, it relates to a method for making a probe apparatus equipped with buckling beam probes.
Integrated circuit dies are sometimes defective and it is undesirable for economic reasons to package defective dies as packaging often represents an expensive step in the manufacture of integrated circuits. Therefore, integrated circuit (IC) dies should be tested before they are packaged. Very small probes are used to establish electrical contact with the pads of an IC die. The probes are used to apply test voltages at the pads for testing the response of the IC die to determine whether it is defective. IC dies which pass the test are packaged and defective IC dies are discarded.
Buckling beam probe assemblies are often used in the integrated circuit industry to make electrical contact with an integrated circuit die under test. U.S. Pat. No. 4,027,935 to Byrnes et al. describes an exemplary buckling beam probe assembly according to the prior art.
FIG. 1 shows a buckling beam probe assembly according to the prior art. The assembly has buckling beam probes 1 disposed within a lower die 2 and an upper die 3. Each probe 1 has a head 4, a tip 5, and a flexible middle section 6. The heads 4 are positioned by the upper die 3 to contact electrodes 7 of a space transformer 8. The space transformer 8 is in electrical communication with remote testing circuitry. The tips 5 are positioned by the lower die to align with contact pads (not shown) of an integrated circuit (IC) die under test 9. The heads 4 commonly have a rounded surface as shown.
In operation, the IC 9 and tips 5 are brought into contact, and the electrodes 7 and heads 4 are brought into contact. Remote testing circuitry is thus provided with an electrical contact to the IC 9 through the probes 1 and space transformer 8. The flexible middle sections 6 bend so that each probe 1 contacts the IC 9 with approximately the same contact force.
A common problem with buckling beam probe assemblies of this type is that the probes 1 xe2x80x9cfloatxe2x80x9d between the lower die 2 and the upper die 3. That is because in this structure the vertical position of the heads 4 and tips 5 is not accurately fixed; rather, the probes 1 shift over a continuous range of positions between the lower die 2 and the upper die 3. Between testing operations, the probes can settle or drop down by as much as 0.003 inches from the position in which the heads 4 touch the electrodes 7. Also, due to slight variations in friction, the tips 5 do not remain in the same horizontal plane. In other words, tip planarity required for establishing good electrical contact between all probes 1 and the corresponding pads of IC 9 is lost.
Many applications of probe assemblies utilize robots with machine vision to align the probe tips 5 with the IC 9. The machine vision apparatus must image the tips 5 to provide accurate alignment. Unfortunately, for proper imaging, the tips 5 must already lie in the same plane. It is difficult and time consuming to properly position the probe assembly if some of the tips 5 are not located within the focal plane of the machine vision apparatus.
Another problem with buckling beam probe assemblies is that, due to the probe xe2x80x9cfloatxe2x80x9d, the heads 4 impact the electrodes 7 with a substantial velocity. While this velocity may be small, many thousands of impact cycles cause damage to the electrodes 7. This is a problem because space transformers 8 are relatively expensive to replace. Of course, dynamic impacts also damage the heads 4.
To ensure proper electrical contact despite probe float the IC 9 must be contacted with a larger force to thus ensure that all the tips 5 contact the IC 9. Increasing the contact force, in turn increases the forces on the assembly. This situation compounds the above-mentioned problem, since increased contact forces result in high wear rates of electrodes 7 and probes 1. Additionally, high contact forces require that the space transformer 8 be made of strong materials such as ceramics, which tend to be relatively expensive (compared to materials such as polymers).
Presently, a process known as xe2x80x9cbinningxe2x80x9d is used to control the tip planarity and uniformity of the probes 1. Many probes 1 are manufactured and separated into xe2x80x9cbinsxe2x80x9d according to their length. Each probe assembly uses only probes 1 having exactly the same length, i.e., probes from one bin, thereby assuring a certain degree of planarity for the tips 5 and heads 4.
The binning approach presents several problems. First, the binning process is time consuming and relatively expensive. Second, replacing a damaged probe 1 requires a probe from the same bin. Consequently, users of probe assemblies are forced to maintain an inventory of all the probe lengths used. If a suitable replacement probe cannot be found, then all the probes 1 in an assembly must be replaced with probes having identical lengths so that nominal tip planarity is provided.
In view of the above problems, the present state of the art of buckling beam probe assemblies is not satisfactory for low-cost and high-reliability applications. Specifically, the many problems relating to probe tip and probe head planarity are posing a problem in the employment of buckling beam probe assemblies.
Accordingly, it is a primary object of the present invention to provide a method for assembling buckling beam probe assemblies that:
1) produces a probe assembly that has exceptional tip and head planarity;
2) produces a probe assembly that has a reduced amount of xe2x80x9cfloatxe2x80x9d; and
3) produces a probe assembly that does not require the application of large contact forces during regular operation.
Further, it is an object of the present invention to provide a probe assembly that:
1) has exceptional head and tip planarity;
2) has a reduced amount of xe2x80x9cfloatxe2x80x9d;
3) does not require large contact forces for contacting integrated circuits.
These and other objects and advantages will be apparent upon reading the following description and accompanying drawings.
These objects and advantages are attained by the present method for making a buckling beam probe assembly. The probe assembly has a lower die, and upper die and buckling beam probes. The method calls for inserting the probes into the upper die and lower die so that probe tips extend from the lower die and probe heads extend from the upper die. Then, a planar fiducial surface is pressed against the probe tips, causing them to be aligned in a common plane. Preferably, the planar fiducial surface is planar parallel with the lower die and upper die. Then, while the probe tips are in contact with the planar fiducial surface, the heads are abraded or sanded to a plane parallel with the planar fiducial surface. After sanding, the probes have identical lengths from heads to tips.
Preferably, the planar fiducial surface is pressed against all the tips. The probe tips can be pressed against the planar fiducial surface by simply placing the probe assembly on top of the fiducial surface. Also, spacers can be placed between the lower die and planar fiducial surface.
The abrading step can be performed by placing the planar fiducial surface and probe assembly on top of a Z-stage. The Z-stage is located under a top plate of a table. The Z-stage is raised, raising the probe assembly until the probe heads extend above the top plate. Then, an abrasive plate disposed on the top plate is rubbed against the probe heads. Alternatively, the Z-stage can include the planar fiducial surface. In yet another embodiment, the Z-stage can be replaced with a block of material having a predetermined height. The height of the block is selected so that probe heads extend above the top plate when the probe assembly is placed on the block.
Also included in the present invention is a method for abrading probe tips. The method for abrading probe tips is analogous to the method for abrading probe heads. Probe heads are defined in the present specification as the end of the buckling beam probe intended for contact with a space transformer; probe tips are the end of the buckling beam probe intended for contact with the integrated circuit.
The present invention includes probe assemblies made according to the present invention. Probe assemblies made according to the present invention are characterized by probe heads or probe tips with flat, abraded surfaces.